Whenever you check the box to "Remember my credentials" in a Windows Security logon box, your username and password are cached in the Windows Credential Manager.
In Outlook, if you are encountering issues accessing mailboxes other than your own which you know you have access to, the problem can often be resolved by clearing the Outlook cached credentials in the Credential Manager.
To open the Credential Manager click the Start Menu icon in the lower left corner of your Windows screen and type "credential manager" in the Search text box that appears right above it. You should then see the Credential Manager show up in the list of results. Click the Credential Manager icon in this list.